Delayed coking is a process used in the petroleum refining industry for increasing the yield of liquid product from heavy residual oils such as vacuum resid. In delayed coking, the heavy oil feed is heated in a furnace to a temperature at which thermal cracking is initiated but is low enough to reduce the extent of cracking in the furnace itself. The heated feed is then led into a large drum in which the cracking proceeds over an extended period of residence in the drum. The cracking produces hydrocarbons of lower molecular weight than the feed which, at the temperatures prevailing in the drum, are in vapor form and which rise to the top of the drum where they are led off to the downstream product recovery unit with its fractionation facilities. The thermal cracking of the feed which takes place in the drum also produces coke which gradually accumulates in the drum during the delayed coking cycle. When the coke reaches a certain level in the drum, the introduction of the feed is terminated and the cracked products remaining in the drum removed by purging with steam. After this, the coke is quenched with water and then discharged through the bottom of the drum, usually by hydraulic jetting or cutting with high pressure water jets followed by the “unheading” or the opening of the drum discharge valve or chute at the drum bottom. The cracking cycle is then ready to be repeated.
Delayed coking drums are conventionally large vessels, typically at least 4 and possibly as much as 10 m in diameter with heights of 10 to 30 m. or even more. The drums are usually operated in twos or threes with each drum sequentially going through a charge-quench-discharge cycle, with the heated feed being switched to the drum in the feed phase of the cycle. The drums are typically made of unlined or clad steel, from about 10 to 30 mm. thick. In form, the drums comprise vertical cylinders with a lower frusto-conical portion between the upper cylindrical portion and a lower portion of reduced diameter which at its lower extremity has either a bottom closure disk or, alternatively, a mechanical valve arrangement as described, for example, in U.S. Pat. No. 6,843,889 (Lah). The feed and steam inlet or inlets may be located in this lower portion or alternatively, in a drum closure disk which seals off the coke discharge opening at the bottom of the drum.
The coking drum is conventionally supported by means of a skirt which is welded to the drum around the lower periphery of the main cylindrical portion of the drum; the skirt transmits the weight of the drum downwards to the underlying support structure and also resists lateral forces generated by wind or seismic movements.
This conventional welded skirt support has long been recognized as a source of problems. Cracking of the skirt attachment weld has been the most prolific difficulty to the extent that instances have been reported of the drum actually becoming separated from the skirt and being left to sit loosely upon the skirt, as reported in Proc. Am. Pet. Inst. 38 [III], 214-232 (1958) (Weil et al), see especially, page 219. If this occurs, the drum no longer has adequate resistance to lateral movement or loading, a situation which cannot long be allowed to continue.
A number of factors contribute to the weakness in the weld in this area, a problem which appears to be largely unique to coking drum design and not shared by other process tower installations, as noted by Weil (page 218). First, the heating and quenching characteristic of the process, recurring at intervals of 12-24 hours, produces repeated expansion and contraction cycles in which the drum movement may not be replicated in the skirt because the skirt has a relatively large air-cooled surface area so that it remains at a temperature below that of the drum rather in the manner of the handle on a skillet. Hoop stresses are generated with resulting weld stress leading to eventual failure. In addition, lateral forces on the drum transferred to the skirt through the weld induce transverse weld stress which may literally crack the weld and open a gap between the skirt and the drum. Aside from these problems, geometric discontinuities and failure to properly relieve weld stresses may accelerate weld failure in the already stressful environment. In the industry, these problems have led over the years to considerable analysis and consideration of techniques for improvement of the weld between the skirt and the drum but, prior to the present invention, no satisfactory solution has been achieved.